This invention relates to sheathing materials for alkaline earth metal-copper oxide based ceramic superconducting materials, such as YBa.sub.2 Cu.sub.3 O.sub.7-x, where the sheathing contains an alloy layer having dispersed particles and has high strength and high temperature characteristics as well as thermal expansion properties compatible with the superconducting materials.
Perovskite related ceramic oxides, comprising alkaline earth metal-copper oxide, such as orthorhombic yttrium-barium-copper oxide materials, usually characterized as YBa.sub.2 Cu.sub.3 O.sub.7-x, or "1:2:3 ceramic oxides", are well-known "high temperature" superconductor materials. This 1:2:3 ceramic oxide material has been found to provide electrical superconductivity, that is, essentially no electrical resistance, at temperatures less than or in the region of 93.degree. K.
The 1:2:3 ceramic oxides and other rare earth metal-alkaline earth metal-copper oxide based ceramics can operate in the superconducting mode near the 77.degree. K. boiling point of relatively inexpensive and plentiful liquid nitrogen, and could find increased use in composite windings for high current magnets, energy storage coils, long distance power transmission, and the like. However, 1:2:3 ceramic oxides and other superconducting ceramic oxides, generally made from sintered component oxide particles, are hard and brittle, and are not easily fabricated into fine wire or windings.
This brittleness was recognized by Jin et al., in Applied Physics Letters, .cent.High T.sub.c Supercontuctors-Composite Wire Fabrication", Vol, 51, No. 3, July 20, 1987, pp. 203-204. Jin et al. also recognized the problem of oxygen loss from the metal clad 1:2:3 ceramic oxide. As a solution to these problems, Jin et al. placed a metal cladding around a heat treated 1:2:3 ceramic oxide powder superconducting core. The metal cladding, which was Ag, or Cu with a Ni/Au oxygen diffusion barrier, allowed ease of drawing into fine wire form, from 0.6 cm to 0.02 cm diameter, and also provided an alternate electrical conduction path in case the ceramic oxide lost its superconducting properties, that is, became normal or resistive. Ag was found particularly advantageous as a cladding. The drawn wires were annealed to 900.degree. C. in oxygen. Multifilamentary ribbon composites were also formed.
Fabrication of Ag sheather, 1:2:3 ceramic oxide superconductors such as wires and tapes has been achieved, but in order to attain or restore optimum superconducting properties, it has been found essential to perform final high temperature annealing in oxygen to optimize the core stoichiometry. This post processing heat treatment is limited in maximum temperature by the 960.degree. C. melting point of silver. Thus, post processing heat treatment typically takes place at around 900.degree. C., instead of much closer to the approximate 1,040.degree. C. melting point of the 1:2:3 ceramic oxide.
Use of other higher melting point metals is limited by their poor oxygen diffusivity characteristics. This limitation in maximum heat treatment temperature results in less than optimum sintered densities in the core. Also, the differential thermal expansion between the silver sheath, 19.7.times.10.sup.-6 /.degree.C., and the 1:2:3 ceramic oxide core, 14.times.10.sup.-6 /.degree.C., may cause the silver sheath to expand away from the core during heat treatment. This can result in interruption of the electrical connectivity. Finally, the silver sheath, which will be subjected to substantial mechanical strain during drawing into thin wire and other fabrication steps, doesn't possess high strength characteristics.
In an attempt to provide high strength sheathing for superconducting core material, in order to alleviate distortion of sheathed superconducting filaments, U.S. Pat. No. 4,863,804 (Whitlow et al.) taught composite sheathing, where a copper or aluminum inner layer was dispersion hardened with from 0.01 weight % to 1 weight % of Al.sub.2 O.sub.3, ZrO.sub.2, SiO.sub.2, TiO, Y.sub.2 O.sub.3, Cr.sub.2 O.sub.3, Th.sub.2 O.sub.3, SiC or BC. Alloys, such as Cu-Nb, Cu-Ta, or Al-Fe were also taught as effective. The highly ductile outer layer could be high conductivity copper or aluminum.
Flukiger et al., in Physica-C, "Metallurgy and Critical Currents in YBa.sub.2 Cu.sub.3 O.sub.7 Wires", 153-155, 1988, pp. 1574-1579, also recognized deformation problems with fine superconducting wire resulting in a decrease of critical temperature, T.sub.c, properties, and the low annealing temperatures dictated by Ag sheathing. Their solution was a sheath containing Ag with up to 5 atom % Pd.
Similarly, Flukiger, in International (German) Patent Application WO 89/02656, filed on Sep. 2, 1988, taught that with an increasing degree of deformation of sheathed YBa.sub.2 Cu.sub.3 O.sub.7 superconducting wire, the critical current density, J.sub.c, decreased, and that with heavy deformation, superconducting properties could disappear. In an effort to provide a sheath material capable of being heat treated at temperatures within the range of 940.degree. C. to 1,030.degree. C., wherein T.sub.c values could be significantly increased after superconducting wire deformations, Flukiger taught alloying Ag with: up to 35 weight % Au, up to 31 weight % Mn, up to 20 weight % Pd, up to 35 weight % Pt, or up to 3 weight % Ti. These alloys provided melting points of from 1,000.degree. C. to 1,070.degree. C. Values of J.sub.c =450A/cm.sup.2 for 1:2:3 ceramic oxides in a sheath of 92 weight % Ag-8 weight % Pd annealed at 970.degree. C.; and values of J.sub.c =680 A/cm.sup.2 for 1:2:3 ceramic oxides in a sheath of 80 weight % Ag-20 weight % Pd annealed at 990.degree. C. were reported. These values are improved over the published value of J.sub.c =175A/cm.sup.2 for a 100% Ag sheath for 1:2:3 ceramic oxides, recovery annealed at 900.degree. C.
None of these superconductor sheaths, however, provide superior high strength and high temperature characteristics, as well as thermal expansion properties compatible with the superconducting materials. It is one of the main objects of this invention to provide such sheaths and superconducting wire.